1. Field of the Invention
The present invention relates to an electrostatic latent image developer and an image forming method used for developing an electrostatic latent image in electrophotography and electrostatic recording.
2. Description of the Related Art
A method for visualizing image information through an electrostatic latent image such as an electrophotography method and the like is currently used in various fields. In the electrophotography method, a latent image is formed on a photosensitive member as a latent image holder by a charging process and an exposing process, the latent image is visualized in a developing process, and a toner image visualized via a transferring process and a fixing process is fixed on an image recording body such as paper and the like. As a developing method in the developing process, a cascade method and the like have formerly been used, and recently, a magnetic brush method has become the most common method. This method uses a magnetic roll as a member for transporting a developer since it is excellent in reproduction not only of a line image but also a solid image and a continuous gradation.
The electrostatic latent image developers used in the developing process are widely classified into two-component developer comprises a toner and a carrier, and single-component developers comprises only a toner. The two-component developer is currently widely used due to its excellent controllability obtained through the functions of an electrostatic latent image developer being separately performed by the toner and the carrier.
Generally the carriers in the two-component developer are largely divided into coated carriers in which a resin-coated layer is formed on the surface of a magnetic powder such as iron, ferrite, magnetite and the like, and into non-coated carriers in which no resin-coated layer is formed on the surface of a magnetic powder. This coated carrier is often used due to the excellent controllability of electric resistivity and the long life span of the developer, and various types of coated carriers have been developed and used in practical applications.
On the other hand, the toners are usually composed of additives and non-magnetic resin particles containing a binding resin, a coloring agent and a releasing agent, and in the case of a black toner, carbon black is usually used as the coloring agent. The additives are used to prevent a decrease in adhesion between the toner and the carrier, to prevent flocculation of the toner, to improve toner flowability, to control static electricity, and the like. They are dispersed on the surface of non-magnetic resin particles. Further, metal oxide particles are usually used as additives, and a surface treatment such as hydrophobitization and the like is effected on the particle to control the electrostatic properties, environmental stability, resistivity and the like.
The raw material, characteristics, structure, composition and the like of the toner and carrier used in two-component developer are selected depending on conditions in the developing apparatus such as the electric field, transportation, magnetic field, stress and the like.
In the developing method using a two-component developer, it is usual to rotate a magnetic roll and photosensitive member in a forward direction and make the circumferential speed of the magnetic roll faster than that of the photosensitive member, in order to guarantee sufficient image density, namely, in order to supply sufficient electrostatic latent image developer in the developing area. However, in this method, developing defects occur due to the difference in relative speeds between the magnetic roll and the photosensitive body, for example, the rear edge of a solid image being left void, and the rear edge of a half tone image in the border region between the front edge of a solid image and the half tone being left void in cases when the half tone and the solid image simultaneously exist. These image voids occur since changes in the amount of electric potential in the electrostatic latent image developer layer, arising from movement of the toner in the developing nip area, depend on the latent image structure. In the above-described method, it is supposed that, since the area to be developed is developed by using an electrostatic latent image developer which has received the influence of the electric field of the area directly in front of this area, the above-described defects are noticeable in areas where the latent image is non-continuous, for example, the border between a solid image part and a part with no image, or the border between half tone and a solid image.
As an example of has to improve these defects, suppressing the volume specific resistivity of a carrier at low levels is suggested in Japanese Patent Application Laid-Open (JP-A) No. 5-61271, Japanese Patent Application Publication (JP-B) Nos. 7-31422, 7-120086 and the like. Further, to lower the volume specific resistivity of a carrier, a method in which the surface of a magnetic powder is coated with a resin to which an electroconductive component is added, is suggested in Japanese Patent application Laid-Open (JP-A) No. 56-75659 and the like.
However, the inventions described in the aforementioned publications do not prevent carrier-over caused by the injection of an electric charge into a carrier, or what are called brush marks which are caused by the generating of latent image leaks, or lowering the ability to supply toner to a photosensitive body due to the proximity of a developing effective electrode to the photosensitive body, and the like, which occur when the volume specific resistivity of a carrier is excessively lowered.
Further, in the inventions described in the aforementioned publications, fogging caused by the injection of electric charge occurs, depending on the toner used, even when a carrier having volume specific resistivity in the range which does not cause the above-described defects in an electrostatic latent image developer is used. This phenomenon has become more remarkable through the use of carriers having volume specific resistivity in the range which does not cause the above-described defects, or through the use of developing systems in which AC bias and DC bias are used simultaneously.